Fuel systems for gas turbine engines meter the flow of fuel so that the required quantity is provided at all engine operating conditions. One or more fuel pumps are used to deliver fuel to the fuel spray nozzles which inject it into the combustion system in the form of atomised spray. For simplicity fixed positive displacement pumps such as gear pumps are used to deliver the fuel to the spray nozzles. The gear pumps have a fixed displacement and are driven by the engine gear train. The output of the pump is therefore directly proportional to the speed of the engine. The fuel flow to the spray nozzles is controlled by recirculating any fuel in excess of the engines requirements back to the pump inlet. A valve, sensitive to the pressure drop across the controlling units, in the system opens and closes as necessary to increase or decrease the amount of fuel recirculated.
A problem with fuel pumping systems which utilise fixed displacement pumps is that the pumps have to supply sufficient fuel for maximum thrust and so have a very high excess capacity at idle. This excess capacity results in the fuel reaching high temperatures. Variable speed drives or variable capacity pumps could be used to reduce the temperature rise however these would introduce mechanical complexity and unreliability.